To ensure both electrical performance and yield of highly integrated circuits, the integrated circuits have to be manufactured with high accuracy and reproducibility. This means that their geometrical dimensions, especially of transistors, lines and via holes, have to be kept within tight tolerances. With increasing integrated circuit integration and correspondingly decreasing feature sizes of the structures involved, tolerances have become narrower and extremely critical. To maintain electrical performance and yield, consequently, each process step in semi-conductor production has to be controlled by "Critical Dimension (CD) Measurement Tools" and "Inspection Microscopes". In the past, these tools were mainly light-optically based, such as a light-optical CD measurement system and a light-optical review station. At present, however, scanning electron microscopes are used to handle the small dimensions of the structures during their production. State of the art feature size is 0.25 .mu.m and below, which requires a measurement accuracy of 20 nm and below, and the number of layers is increasing, i.e. 5 to 7 layers. Since such fine and multi-layer structures are "invisible" for light-optical equipment, scanning electron microscopes are now used for this purpose.
However, with decreasing structure sizes, even measurements based on scanning electron microscopes are reaching their limits. This limitation is not related to spatial resolution but to visibility and dimension measurement of structures with high aspect ratio. Semi-conductor technology requires a certain height of its structures (e.g. resist thickness, metal and oxide thickness). The aspect ratio (height/width of the structure) becomes increasingly larger. This especially applies to contact holes (via holes) having aspect ratios larger than 5 (0.2 .mu.m hole in 1 .mu.m resist layer).
Due to this high aspect ratio the visibility and accordingly the measurement of critical dimensions at the bottom of the structure, which is extremely important for the device characteristic, becomes difficult and in many cases impossible.
The reason for this invisibility is the secondary electrons released by the primary electron beam, which are difficult to detect. In post-lens detection systems, where the detector is arranged between the probe and the lens, the secondary electrons cannot be extracted from the bottom of the structure. In-lens or pre-lens detection systems, where the detector is arranged in or in front of the lens, use a high secondary electron attraction field causing the secondary electrons from the bottom of the structure to occupy only a small angle close to the optical axis and to move up the scanning electron microscope column in the direction of the cathode. Since the secondary electrons are extracted and accelerated, they behave very much like the primary electron beam and, therefore, are difficult to detect. Additionally, surface charging can influence the emission and detection of the secondary electrons.
The present solutions to overcome these problems are:
use of backscattered electrons for image generation, PA1 positive surface charging of the upper part of the structure to extract the secondary electrons from the bottom of the structure, PA1 use of a beam separator for the primary beam and secondary electron or backscattered electron beam, e.g. a Wien filter; this, however, requires an additional optical element in the microscope, which may influence the spatial resolution of the instrument. PA1 1. At least parts of the material surrounding the structure are removed to a certain depth in order to decrease the aspect ratio. PA1 2. An etching mask for the removal of the surroundings is generated, which avoids damage to the interesting feature and which overlaps the structure partially. The removal of the material will then open gateways for the backscattered and secondary corpuscles.
Backscattered electrons and accelerated secondary electrons are, as mentioned, difficult to detect and require additional elements with consequent limitation of resolution. Artificial surface charging also causes disturbances of the primary beam and consequently causes measurement errors or limitations.
It is an object of the present invention to provide a method and an apparatus for dimension measurement and inspection of structures having a high aspect ratio, without limiting spatial resolution and without causing disturbances of the primary beam.